NBC’s top brass are worried about Jimmy Fallon‘s “Late Night” show after the ratings dropped 20 percent in a year.

Recent Nielsen figures show Fallon’s viewers have dropped to 1.63 million — a 20 percent fall from May 2009 — and he’s being beaten by both his ABC rival Jimmy Kimmel and CBS’s Craig Ferguson, whose May 2010 ratings were 1.72 million and 1.7 million, respectively.

The Peacock Network is trying to boost Fallon’s audience by booking appearances on his show by NBC and CNBC personalities, including Brian Williams, Rachel Maddow and Meredith Vieira.

A TV industry insider told Page Six, “NBC executives are concerned about the ratings drop and are doing all they can to promote Fallon . . . They’re even dragging him out of bed in the morning to do the ‘Today’ show to promote his own show.”

Fallon’s ratings decline is further fallout from the late-night debacle with Conan O’Brien and Jay Leno.

Marc Berman, Mediaweek’s “Mr. Television,” told us, “When they took out Jay and put in Conan, they lost a lot of the viewers who would usually have stayed on to watch Fallon. The lead-in is pivotal . . . When you mess around with viewers, sometimes it’s hard to get them back. TV is a very fickle business . . . If there is a lesson to be learned from this debacle, it is that if it’s not broken, don’t fix it.”

But an NBC source claimed, “[Fallon’s] ratings in ’09 were unusually high because there was a big lead-in from Leno’s farewell shows, so it is unfair to compare that to this May. We think the drop in ratings is much less, around 12 percent.

“The network is more interested in the key demographic, the 18- to 49-year-olds, among which Fallon beats Ferguson and Kimmel night after night.”

An NBC rep said, “We are huge Jimmy Fallon fans and could not be happier with his performance. Fallon continues to win night after night in the key demographic.”